Horsepower and wattmeters have been used in the past to determine the power dissipated in electrical loads like large electrical motors used in machine tools such as lathes, machining centers, grinders, etc. Monitoring the power dissipation in such motors is useful for indicating tool wear, recording operational parameters, maximizing operational efficiencies, preventive maintenance and the like. Historically, wattmeters have been of the galvanometer type which make use of the magnetic interaction between a moving electromagnet on the pointer of the meter and stationary electromagnets. The multiplication of current and voltage necessary to determine power is accomplished by mechanically passing these two coils through one another's electromagnetic fields. Unfortunately, this type of meter has a number of drawbacks. First, perfect coupling cannot be achieved between the electromagnetic fields and, thus, inaccuracies are introduced into the meter. Secondly, this type of meter does not lend itself well to interfacing with electronic equipment, such as process controllers or recorders, due to the heavy current which is necessarily passed through the meter.
A more modern type of wattmeter uses the Hall effect principle in an integrated circuit surrounded by the same type of electromagnets as in the galvanometer wattmeter. This type of wattmeter can be interfaced with electronic equipment, but shares another drawback with the galvanometer. Both the galvanometer and Hall effect meter use heavy windings (electromagnets) within the instrument. When the current flow is too great for the meter's internal windings to carry, an external current transformer is normally used which introduces greater cost, greater instrument error and restricts use to AC applications. Other drawbacks are the large physical size of the wattmeter and the potential danger to the user unless suitable precautions are taken.
The power monitoring device disclosed in U.S. Pat. No. 4,096,436 to Cook et al overcomes many of the problems of the earlier approaches. In the '436 patent there is disclosed a device which uses a current shunt wired in series with the load. In operation, the voltage drop across the shunt is proportional to the current flowing through the load and the monitor multiplies this signal with a signal related to voltage drop across the load to form an output signal related to power. This output signal can be used to drive a display of the instantaneous power being drawn by the load or can be wired to an external device such as a process controller like that disclosed in U.S. Pat. No. 4,279,013 to Cameron et al.